There are various known methods of treating blood extra-corporeally such for example as haemodialysis, haemofiltration and the combination of these two methods which is referred to as haemodiafiltration. During haemodialysis treatment, the patient's blood and a dialysis fluid flow across the semi-permeable membrane which divides the dialyser into the blood and dialysis-fluid chambers, preferably in counter-current and each at a preset flow rate.
To optimise the methods of blood treatment, it is known for parameters characteristic of the blood treatment to be determined. These characteristic parameters include in particular the parameters which define the metabolic performance of the dialyser.
The metabolic performance of the dialyser can be defined by clearance, which, for a given substance, means that virtual volume of blood which, per minute, is completely freed of the substance in question by the dialyser. Dialysance is a further term for use in determining the performance of the dialyser and in its case account is also taken of the concentration in the dialysis fluid of the substance involved in the metabolic exchange in the dialyser.
Clearance or dialysance depends in turn, to crucial degree, on the preset blood flow rate at which the patient's blood flows through the blood chamber of the dialyser. As the blood flow rate increases so too does the clearance in this case. The blood treatment should therefore take place at as high a blood flow rate as possible. However, in practice there are found to be limits on how far the blood flow rate can be increased.
An upper limit on the blood flow rate is set by the vascular access, which puts a limit on the arterial inflow pressure and venous return pressure in the blood lines. An increase in the blood flow rate at which the blood pump pumps the blood through the blood lines results in an increase in the suction pressure from the blood pump, which may cause the vessel to collapse or the puncture needle to be sucked against the wall of the vessel. If this happens, an arterial pressure alarm is triggered. The limiting values for the blood flow rate differ from patient to patient as a function of the characteristics of the vascular access. Even on the same patient different limiting values arise for the blood flow rate because, for example, recently fitted shunts (arterio-venous fistulas) are generally still relatively unstable and will not stand excessively high pressures. Current maximum values for the inflow pressures are in the range from −200 mmHg to −150 mmHg, and for the return pressures figures of up to +150 mmHg to +200 mmHg are tolerated. The level of the blood flow rate is also of crucial importance in the case of the puncture needles because needles of small diameter set up a greater resistance to flow than ones of a large diameter.
To date, the blood flow rate has been set manually with due allowance for the preset parameters. What is often habitually set is a value which has proved satisfactory in practice, such for example as 300 ml/min, 350 ml/min or 400 ml/min, even though the blood and the patient's physiology would, fundamentally, allow values different from this to be set.